That is a cost, but it is a cost determined by each monk himself/herself.

No it isn't. If perlmonks gets damaged we all suffer.

If I would like to spend my time help this guy with c++ question for free, why is it your business?

Because you're making nodes. If you want to talk, /msg him .

Yes, this is a Perl site, but people can occasionally ask c++ question especially if he/she is a member of this site, as I believe that he will ask more questions about perl than anything else.

Not unless its about perl. There is no c++ only section.